perf: arm_cspmu: Fix module cyclic dependency

perf: arm_cspmu: Fix module cyclic dependency

Build on arm64 allmodconfig failed with:
  | depmod: ERROR: Cycle detected: arm_cspmu -> nvidia_cspmu -> arm_cspmu
  | depmod: ERROR: Found 2 modules in dependency cycles!

The arm_cspmu.c provides standard functions to operate the PMU and the
vendor code provides vendor specific attributes. Both need to be built as
single kernel module.

Update the makefile to compile sources under arm_cspmu into one module.

Signed-off-by: Besar Wicaksono <bwicaksono@nvidia.com>
Reviewed-and-Tested-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/20221116203952.34168-1-bwicaksono@nvidia.com
Signed-off-by: Will Deacon <will@kernel.org>